CBSE Class 8 Science Chapter 02 The Invisible Living World Beyond Our Naked Eye MCQs Set 03

Science Objective Questions and Answers: Chapter 02 The Invisible Living World Beyond Our Naked Eye

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Question: In Activity 2.1, when a round-bottom flask filled with water is placed over printed text, the letters appear magnified. Which explanation best describes the physical principle behind this magnification?
A. The water inside the flask bends light rays passing through it, causing objects on the other side to appear larger
B. The glass flask absorbs certain wavelengths of light, making the text stand out more clearly
C. The curved surface of the flask acts as a barrier that prevents the text from being seen at its actual distance
D. The water evaporates and creates a lens-like effect that focuses light onto the printed page
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Answer: (A) The water inside the flask bends light rays passing through it, causing objects on the other side to appear larger

Explanation:
A curved container filled with a transparent medium like water functions as a simple lens by refracting (bending) light rays. This bending causes light rays to converge, making objects appear larger when viewed through it—the same principle that led ancient scientists to discover curved glass could magnify small things.

Question: According to the chapter, what fundamental characteristic makes cells the 'basic unit of life' for all living organisms?
A. Cells are the only structures capable of containing both water and minerals simultaneously
B. All living organisms, whether single-celled or multicellular, are composed of one or more cells, and each cell performs essential life functions
C. Cells are the largest structures visible under a standard school microscope without special preparation
D. Cells must always contain a nucleus in order to be considered truly alive
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Answer: (B) All living organisms, whether single-celled or multicellular, are composed of one or more cells, and each cell performs essential life functions

Explanation:
The chapter emphasises that cells are the building blocks of all living things—from unicellular bacteria to multicellular plants and animals. Every organism performs its life processes at the cellular level, making the cell the fundamental unit of life.

Question: In Activity 2.3, human cheek cells appear as polygon-shaped structures, while onion peel cells from Activity 2.2 appear nearly rectangular. What does this difference in shape primarily indicate?
A. Cheek cells are not real cells because they lack a standard geometric form
B. The shape of cells is determined by their specific location and function in the organism
C. Animal cells are inherently more flexible and change shape frequently throughout the day
D. The microscope used to observe cheek cells magnifies shapes differently than the one used for onion cells
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Answer: (B) The shape of cells is determined by their specific location and function in the organism

Explanation:
Cell shape varies across different cell types and relates to their specialised functions. Rectangular onion peel cells are tightly packed to form protective tissue, while polygonal cheek cells fit together to line the inner mouth. The shape always relates to the cell's role in the organism.

Question: Why did Robert Hooke, observing cork tissue under his microscope in 1665, choose to call the tiny structures he saw 'cells'?
A. The structures were extremely small, like cells in a prison
B. The empty compartments he observed resembled the hexagonal compartments of a honeycomb
C. He wanted to honour the cellular theory proposed by earlier scientists
D. The term was already used in medical texts to describe small body structures
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Answer: (B) The empty compartments he observed resembled the hexagonal compartments of a honeycomb

Explanation:
Hooke drew what he saw and noticed that the small, empty spaces in the cork tissue resembled the compartments of a honeycomb—organised, regular structures. This visual similarity inspired him to adopt the word 'cell' to describe these fundamental units.

Question: What structural feature distinguishes bacterial cells from plant, animal, and fungal cells?
A. Bacteria are the only cells that possess a cell membrane
B. Bacteria lack a well-defined nucleus and nuclear membrane, instead having a nucleoid region
C. Bacteria are larger than other cell types and require special microscopes to observe
D. Bacteria possess a cell wall but no cytoplasm, making them structurally simpler
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Answer: (B) Bacteria lack a well-defined nucleus and nuclear membrane, instead having a nucleoid region

Explanation:
• Bacteria do not have a membrane-bound nucleus
• Instead, their genetic material is located in a nucleoid region
• This distinguishes them fundamentally from eukaryotic cells (plant, animal, fungal) which have a true, membrane-bound nucleus.

Question: In Activity 2.8, dough containing yeast rose and became fluffy after several hours, while dough without yeast remained flat. Which process is primarily responsible for this observation?
A. Yeast chemically combines with flour to increase its volume
B. Yeast respires and breaks down sugar, releasing carbon dioxide gas that forms bubbles in the dough
C. The warm environment causes water in the dough to evaporate and create pockets of air
D. Yeast produces a sticky substance that traps air bubbles from the surrounding atmosphere
Show Answer & Explanation

Answer: (B) Yeast respires and breaks down sugar, releasing carbon dioxide gas that forms bubbles in the dough

Explanation:
Yeast is a fungus that undergoes respiration when provided with sugar and warm conditions. During this metabolic process, yeast breaks down the sugar and releases carbon dioxide gas. These gas bubbles become trapped in the dough, making it soft, fluffy, and expanded.

Question: Based on Activity 2.9, in which curd formation occurred in warm milk (bowl A) but not in cold milk (bowl B), what does this observation most directly demonstrate?
A. Lactobacillus bacteria are only capable of surviving in warm environments
B. Temperature significantly affects the rate at which bacteria like Lactobacillus can multiply and ferment milk
C. Cold temperatures destroy the nutrients in milk, preventing bacterial growth
D. Curd cannot form naturally without heat being applied to the milk first
Show Answer & Explanation

Answer: (B) Temperature significantly affects the rate at which bacteria like Lactobacillus can multiply and ferment milk

Explanation:
Lactobacillus grows and multiplies much faster in warm conditions, fermenting the milk to produce lactic acid and forming curd within hours. In cold temperatures, the bacteria's metabolic activity slows dramatically, so curd formation is delayed or minimal. This illustrates the dependence of microbial growth rates on optimal temperature.

Question: Why did Activity 2.2 require the application of safranin stain before observing onion peel cells under a microscope?
A. Safranin permanently changes the cell structure so that more details become visible
B. The stain provides colour contrast, making the nucleus, cytoplasm, and cell membrane distinguishable from each other
C. Safranin kills the cells in a way that reveals their internal components more clearly
D. The red colour of safranin is the only way to confirm that the observed structures are actually cells
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Answer: (B) The stain provides colour contrast, making the nucleus, cytoplasm, and cell membrane distinguishable from each other

Explanation:
Most cells are naturally transparent and colourless, making them difficult to see under a microscope. Safranin, a red-coloured dye, selectively colours specific cell components, providing visual contrast that allows students to clearly distinguish the nucleus, cytoplasm, cell membrane, and cell wall.

Question: In Activity 2.7, when fruit and vegetable peels were mixed with soil and left for 2–3 weeks, they transformed into dark-coloured, nutrient-rich manure. Which organisms are primarily responsible for this transformation?
A. Only bacteria present in the soil break down organic matter through a process called decomposition
B. Fungi and bacteria work together to decompose plant waste into simpler, nutrient-rich substances
C. The heat from the soil naturally causes chemical reactions that convert waste into manure
D. Earthworms and other visible soil organisms physically break down the peels into smaller pieces
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Answer: (B) Fungi and bacteria work together to decompose plant waste into simpler, nutrient-rich substances

Explanation:
The chapter specifies that microorganisms like fungi and bacteria act on plant waste, slowly breaking it down—a process called decomposition. These microbes convert complex organic substances into simpler, nutrient-rich compounds that enrich the soil.

Question: According to the chapter, which statement about Rhizobium bacteria in legume root nodules is correct?
A. These bacteria steal nitrogen from surrounding soil, reducing fertility for other plants
B. The bacteria trap atmospheric nitrogen and make it available to the plant, improving soil health without chemical fertilisers
C. Rhizobium bacteria only survive in legume plants and cannot exist elsewhere in the environment
D. The relationship between legumes and Rhizobium is parasitic, harming the plant's growth
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Answer: (B) The bacteria trap atmospheric nitrogen and make it available to the plant, improving soil health without chemical fertilisers

Explanation:
Rhizobium bacteria live symbiotically in swollen regions called root nodules on legume roots. They convert atmospheric nitrogen into a form the plant can use, improving plant growth and naturally enriching soil nitrogen levels. This is why farmers practise crop rotation with legumes.

Question: What does the chapter indicate about why pickles and murabbas do not typically become infected by microorganisms?
A. These foods are always kept in sealed containers that microorganisms cannot penetrate
B. The high concentration of salt or sugar in these preserved foods inhibits microbial growth
C. Heat treatment during preparation kills all microorganisms that would otherwise colonise the food
D. Microorganisms simply prefer fresh fruits and vegetables over fermented ones
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Answer: (B) The high concentration of salt or sugar in these preserved foods inhibits microbial growth

Explanation:
The chapter explicitly states that high concentrations of salt or sugar create conditions unfavourable for microbial growth, acting as natural preservatives. This osmotic environment prevents the organisms from multiplying on the food.

Question: In the context of the levels of organisation described in section 2.2, which sequence correctly orders these levels from simplest to most complex?
A. Organism → Organ system → Organ → Tissue → Cell
B. Cell → Tissue → Organ → Organ system → Organism
C. Tissue → Cell → Organ → Organ system → Organism
D. Organ → Cell → Tissue → Organism → Organ system
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Answer: (B) Cell → Tissue → Organ → Organ system → Organism

Explanation:
The chapter clearly states the correct hierarchy: cells are the basic units, similar cells form tissues, tissues combine into organs, organs work together in organ systems, and organ systems comprise a complete organism.

Question: What does the chapter's discussion of Spirulina in the 'Ever heard of' section primarily suggest about the practical applications of microorganisms?
A. Microorganisms are only useful in laboratory settings for scientific research
B. Some microorganisms can be farmed and harvested as food supplements, representing a feasible livelihood opportunity
C. Spirulina is the only microorganism that humans have successfully cultivated for food
D. Microalgae are toxic unless they undergo chemical processing to become safe for consumption
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Answer: (B) Some microorganisms can be farmed and harvested as food supplements, representing a feasible livelihood opportunity

Explanation:
The section explicitly describes Spirulina as a superfood rich in protein and vitamin B12, and notes that its farming 'is becoming a feasible livelihood opportunity.' This demonstrates that microorganisms can be directly cultivated and harvested for human benefit.

Question: Which statement best describes the role of glycerin in Activity 2.2 when preparing onion peel cells for microscopic observation?
A. Glycerin chemically dissolves the cell wall, making internal structures more visible
B. Glycerin prevents the stained cells from drying out and maintains clarity for better visualisation
C. Glycerin neutralises the safranin stain to prevent the cells from being permanently coloured
D. Glycerin hardens the cell membrane, making it easier to see under magnification
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Answer: (B) Glycerin prevents the stained cells from drying out and maintains clarity for better visualisation

Explanation:
The chapter states that glycerin prevents drying of the cells and improves clarity for better visualisation. By maintaining moisture, glycerin preserves the cell structure and prevents deterioration during observation.

Question: Based on Activities 2.4 and 2.5, what conclusion can be drawn about the distribution of microorganisms in nature?
A. Microorganisms are found only in water and not in soil or air
B. Microorganisms exist in diverse environments—in both pond water and soil suspension—indicating they are widespread throughout nature
C. Soil contains no living organisms because microorganisms cannot tolerate dry conditions
D. Only certain types of microorganisms exist in water, while completely different types inhabit soil
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Answer: (B) Microorganisms exist in diverse environments—in both pond water and soil suspension—indicating they are widespread throughout nature

Explanation:
Both activities revealed tiny organisms in their respective samples. The presence of similar moving organisms in both pond water and soil suspension suggests that microorganisms are ubiquitous in nature, thriving in multiple environments.

Question: In Activity 2.8, why was it specifically important to add both sugar and warm water to the flour before adding yeast, rather than simply mixing yeast with dry flour?
A. Sugar and water together create a chemical reaction that produces the gas needed to rise the dough
B. Yeast requires sugar as food and warm conditions to actively respire, break down the sugar, and release carbon dioxide
C. Dry flour would absorb all the yeast and prevent any fermentation from occurring
D. The combination of sugar and water dissolves the gluten in flour, allowing gas bubbles to form more easily
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Answer: (B) Yeast requires sugar as food and warm conditions to actively respire, break down the sugar, and release carbon dioxide

Explanation:
Yeast is a microorganism that must consume sugar and requires warm conditions to respire effectively. The chapter explains that yeast 'breaks down food to release energy for their growth' and that it 'grows well in warm conditions.' Without both sugar and warmth, fermentation cannot occur efficiently.

Question: According to the chapter's explanation of microorganism diversity, which pairing of a microorganism type with an accurate characteristic is correct?
A. Bacteria always contain a visible nucleus enclosed by a nuclear membrane
B. Algae lack chlorophyll and cannot produce their own food through photosynthesis
C. Protozoa are single-celled organisms that move using specialised structures, as shown by Paramecium in Table 2.1
D. Fungi are the only microorganisms capable of decomposing organic matter in soil
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Answer: (C) Protozoa are single-celled organisms that move using specialised structures, as shown by Paramecium in Table 2.1

Explanation:
Table 2.1 describes Paramecium as 'single cell, moves from one place to another, movement takes place with the help of specialised structures.' This accurately characterises protozoa as mobile, single-celled organisms using specialised movement structures.

Question: What inference can be drawn from the chapter's statement that decomposition in Activity 2.7 occurs 'at optimal temperature and appropriate moisture level'?
A. Decomposition is a purely chemical process unrelated to living organisms
B. Environmental conditions directly influence the rate at which microorganisms can break down waste into manure
C. Temperature and moisture are harmful to decomposition and must be minimised
D. Only bacteria can decompose waste; fungi require different conditions and play no role
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Answer: (B) Environmental conditions directly influence the rate at which microorganisms can break down waste into manure

Explanation:
The statement indicates that microorganisms responsible for decomposition have optimal conditions for activity. Temperature and moisture affect how quickly these microbes can multiply and work, directly controlling the speed of waste breakdown and manure formation.

Question: How does the chapter's description of Antonie van Leeuwenhoek earning the title 'Father of Microbiology' reflect the significance of his microscopic observations?
A. He invented the first microscope ever created by humans
B. He was the first person to clearly observe and describe tiny living things like bacteria and blood cells, establishing the field of microbiology
C. He proved that microorganisms were harmful and should be avoided
D. He developed a mathematical theory explaining how microorganisms reproduce
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Answer: (B) He was the first person to clearly observe and describe tiny living things like bacteria and blood cells, establishing the field of microbiology

Explanation:
The chapter states that van Leeuwenhoek 'was the first person to clearly see and describe tiny living things like bacteria and blood cells.' This pioneering work directly led to the establishment of microbiology as a scientific discipline.

Question: Which statement about plant cells and animal cells is directly supported by evidence presented in the chapter?
A. Plant cells are always larger than animal cells
B. Both plant and animal cells contain a nucleus and cytoplasm, but only plant cells have a cell wall and chloroplasts
C. Animal cells have chloroplasts that allow them to make their own food
D. Plant and animal cells are structurally identical except for minor size differences
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Answer: (B) Both plant and animal cells contain a nucleus and cytoplasm, but only plant cells have a cell wall and chloroplasts

Explanation:
• The chapter describes plant cells as having a cell wall and chloroplasts
• Animal cells (such as cheek cells) have nucleus and cytoplasm but lack these structures
• This fundamental difference reflects the different roles and functions of plant versus animal tissues.

Question: What does the chapter's inclusion of a section on Ananda Mohan Chakrabarty's bacterium that decomposes oil spills primarily illustrate?
A. Bacteria are useful only for making food and nothing else
B. Microorganisms can be engineered or selected to solve environmental problems like pollution
C. Oil spills can be completely eliminated without any human intervention
D. Scientific work with bacteria is too dangerous for practical environmental applications
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Answer: (B) Microorganisms can be engineered or selected to solve environmental problems like pollution

Explanation:
The chapter highlights Chakrabarty's work to show how microorganisms can be used to address environmental challenges. His development of a bacterium capable of breaking down oil demonstrates that microbial diversity can be harnessed for solving real-world problems.

Question: In Activity 2.2, when observing onion peel cells under a microscope, the cells appear nearly rectangular and are closely arranged without gaps. What does this tight arrangement of cells primarily reveal about the function of the cell wall in plant structures?
A. It prevents water loss by sealing cells hermetically from the environment.
B. It provides rigidity and strength, allowing cells to maintain firm contact with one another.
C. It enables cells to move freely around the organism in response to stimuli.
D. It facilitates rapid communication between cells through electrical signals.
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Answer: (B) It provides rigidity and strength, allowing cells to maintain firm contact with one another.

Explanation:
The chapter explicitly states that the cell wall 'provides rigidity and strength to plants' and explains that this is 'why all cells are arranged compactly with each other and look firm in structure.' The tight, orderly arrangement observed in the onion peel directly demonstrates this structural support function.

Question: When yeast is added to flour mixed with sugar and warm water in Activity 2.8, the dough rises and becomes fluffy with a distinct smell. Based on the chapter's explanation of yeast fermentation, which sequence best describes what occurs inside the dough?
A. Yeast cells break down sugar molecules, releasing carbon dioxide gas and producing alcohol as byproducts.
B. Yeast cells absorb water from the warm liquid and expand, pushing the dough upward mechanically.
C. Sugar crystallises into larger structures that create air pockets throughout the dough mixture.
D. Warm water causes the flour proteins to denature and release trapped gases from within the grain structure.
Show Answer & Explanation

Answer: (A) Yeast cells break down sugar molecules, releasing carbon dioxide gas and producing alcohol as byproducts.

Explanation:
The chapter explains that yeast 'respires and breaks down food to release energy' and that 'carbon dioxide is released, which forms bubbles that makes the dough soft and fluffy' while 'yeast also produces a small amount of alcohol during this process, which gives the dough a slightly different smell.' This directly matches the first option's description of the fermentation process.

Question: According to the chapter's discussion of organisational hierarchy in section 2.2, how does the relationship between individual cells and a complete organism illustrate why cells are considered the 'basic unit of life'?
A. Cells are the smallest structures visible under a light microscope and therefore represent the practical limit of biological study.
B. Cells serve as the fundamental building blocks from which all living systems are constructed, capable of performing all necessary life functions either individually or in cooperation.
C. Cells are spherical in shape, which makes them the most efficient molecular containers for storing genetic information.
D. Cells must always exist in multicellular groups; individual cells can never survive or function independently in nature.
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Answer: (B) Cells serve as the fundamental building blocks from which all living systems are constructed, capable of performing all necessary life functions either individually or in cooperation.

Explanation:
The chapter establishes that 'a single cell contains various components that help organisms perform various functions' and that cells can work together in multicellular organisms while unicellular organisms 'carry out all the functions necessary for their survival in a single cell.' This dual capacity—functioning alone or cooperating—demonstrates why cells are the fundamental unit underlying all life.

Question: In Activity 2.9, curd formation occurred in warm milk (bowl A) but not in cold milk (bowl B), even though curd starter containing Lactobacillus bacteria was added to both. What does this experimental outcome reveal about the relationship between environmental conditions and microorganism activity?
A. Lactobacillus bacteria are unable to survive in cold temperatures and die when exposed to refrigeration.
B. Microorganism metabolic processes depend on specific environmental conditions; these bacteria function optimally at warm temperatures required for their growth and fermentation activity.
C. Cold temperatures reverse the fermentation process, causing already-formed lactic acid to convert back into lactose sugar.
D. The curd starter used in bowl B must have contained a different bacterial species that cannot tolerate warmth.
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Answer: (B) Microorganism metabolic processes depend on specific environmental conditions; these bacteria function optimally at warm temperatures required for their growth and fermentation activity.

Explanation:
• The chapter states bacteria 'grow well in warm conditions'
• Lactobacillus 'feeds on the sugar in the milk (lactose), multiplies, and ferments the milk to form curd'
• Without the warm environment, metabolic activity slows dramatically, preventing the fermentation process from occurring at a meaningful rate
This demonstrates that microorganism function is directly tied to environmental conditions rather than the bacteria's presence or absence alone.

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